cmd/tailscale: improve taildrop progress printer on Linux (#9878)

The progress printer was buggy where it would not print correctly
and some of the truncation logic was faulty.

The progress printer now prints something like:

	go1.21.3.linux-amd64.tar.gz               21.53MiB      13.83MiB/s     33.88%    ETA 00:00:03

where it shows
* the number of bytes transferred so far
* the rate of bytes transferred
  (using a 1-second half-life for an exponentially weighted average)
* the progress made as a percentage
* the estimated time
  (as calculated from the rate of bytes transferred)

Other changes:
* It now correctly prints the progress for very small files
* It prints at a faster rate (4Hz instead of 1Hz)
* It uses IEC units for byte quantities
  (to avoid ambiguities of "kb" being kilobits or kilobytes)

Updates tailscale/corp#14772

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

cmd/tailscale/depaware.txt

@@ -158,6 +158,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/util/singleflight                              from tailscale.com/net/dnscache
         tailscale.com/util/slicesx                                   from tailscale.com/net/dnscache+
         tailscale.com/util/testenv                                   from tailscale.com/cmd/tailscale/cli
+        tailscale.com/util/truncate                                  from tailscale.com/cmd/tailscale/cli
         tailscale.com/util/vizerror                                  from tailscale.com/types/ipproto+
      💣 tailscale.com/util/winutil                                   from tailscale.com/hostinfo+
    W 💣 tailscale.com/util/winutil/authenticode                      from tailscale.com/clientupdate